'Project: Time Off' surveyed 7,331 workers, aged 18 and older, who work more than 35 hours a week and receive paid time off from their employer.

They found there was a culture of women feeling guilty (23 per cent to 20 per cent of the women surveyed) if they took their days off, as well as being worried it would make them look less committed to their career (28 per cent to 25 per cent).

The other reason they gave is called 'work martyrdom' which is when you don't take time off to impress your boss.

Apparently both male and female millennials are the most likely generation to be work martyrs. This is due, in part, to entering the workforce in economically unstable times and having the highest University debt.

A whopping 44 per cent of millennials want their boss to view them as a work martyr opposed to 37 per cent of 'Generation X' and the 33 per cent of 'Boomers'.

The report noted that, 'Where 51 per cent of millennial men used all their vacation time in 2016 (up from 44 per cent in 2015), just 44 percent of millennial women did (46 per cent did in 2015).'

This was despite the fact that millennial women are more likely than millennial men to say vacation time is 'extremely' important to them (55 per cent to 45 per cent), since they know that taking a holiday will help avoiding burnout (85 per cent to 76 per cent), will boost morale (84 per cent to 76 per cent), improves employee focus (82 per cent to 72 per cent), improves health and well-being (84 per cent to 77 per cent), and renews employees' job commitment (76 per cent to 68 per cent).

These things are all true, so why are they not taking their days off?

According to Fortune, Katie Denis, senior director of 'Project Time Off' who authored the report, says the reasons for the gap are difficult to pinpoint, but one factor could be millennial men's growing professional confidence; they feel secure enough in their jobs to be out of the office sentiments, ultimately preventing them from taking time off.

As much as holiday is effective, work martyrdom is ineffective.

The report stated that people who sacrifice their holiday days are less likely than non-forfeiters to have been promoted within the last year (23 per cent to 27 per cent) and to have received a raise or bonus in the last three years (78 per cent to 84 per cent).

As well as their own career and economic losses, their actual time and work is more likely to be stressful and they become more susceptible to the fears that stop employees from taking time off in the first place.Between 1977 and 2000 there was an average over both sexes of 20 days of holiday a year.

After the Millenium, worker's days off took a swift nose-dive, and despite there being a 0.6 increase in days since last year, the average is a grand total of 16.8 days.

As well as vacation days being good for mental health, physical health and personal career progression, unused vacation days are also good for the economy.